This invention relates to ring trip detection circuits.
In order to address the need to provide more telephone extension lines at the premises of subscribers, whilst minimizing the total length of copper wire used, pair gain systems have become popular wherein several telephone lines are served over a single or double pair of wires from a central office. This single or double pair of wires comes from an exchange unit co-located with the central office, and are connected to a remote unit (RU) near the subscribers. These remote units, also known as network termination units (NTU), are typically powered via a DC voltage which is also fed along those same wires from the exchange unit. The NTU must act as the central office to the attached subscribers, supplying them among other things with DC loop voltage for operation, and an AC ringing voltage when a subscriber is being called. When one of the attached subscriber lines is called, the exchange unit sends a signal to the NTU to ring that subscriber line, and the NTU in turn sends an A.C. ringing voltage to the line for energizing the ringers of the telephone extension units or other equipment connected to that subscriber line, generally known as Customer Premises Equipment (CPE). When someone answers one of the telephone extension units, this must be detected and the ringing voltage removed from the answered telephone line within approximately 200 ms, since otherwise the a.c. ringing voltage creates high volume noise through the telephone earpiece causing significant discomfort to the called subscriber.
In practice, it is most convenient for the NTU to generate one ringing voltage for all connected subscribers at once, and simply connect the ringing voltage to all subscribers who are being called at that instance. As a result, while one subscriber may have answered the line, the ringing signal may still need to be sent to other subscriber lines which are connected to the same NTU. Therefore, whilst the AC ringing voltage must be quickly removed from the subscriber line which goes off-hook, this must be done in a manner which does not perceptibly halt the ring signal to other subscriber lines which are connected to the NTU and which remain on-hook and to which a call is also directed.
Conventionally, this requirement is addressed by providing a ring trip detector in each subscriber line interface circuit (SLIC) so that when the receiver of an attached CPE is lifted (i.e. the telephone extension unit goes xe2x80x9coff hookxe2x80x9d), this is detected and the ring signal is interrupted to the telephone line. However, in a system having multiple telephone lines connected to a single NTU, this multiplies the number of ring trip detectors thus required and increases the overall cost of the telephone system.
A typical prior art ring trip detector circuit is Lucent""s L8551 Low Power SLIC chip which requires that each subscriber be provided with its own separate ring trip detector and off hook detector. The off hook detector on its own cannot function as a ring trip detector owing to the unbalanced nature of the AC ringing voltage relative to ground, thus requiring a separate trip detector for each subscriber line. Owing to the need to conserve power, during the application of the AC ringing voltage, the DC loop current is switched off from all ringing lines.
U.S. Pat. No. 4,447,673 (Elliott et al.) discloses a ring trip detector circuit adapted for use in a community office switching system that includes a call control processor and a port event processor. The call control processor and port event processor interact through a port storage unit that contains information corresponding to each individual telephone line connected to the switching system. The switching system also includes a ringing generator for transmitting ringing signals onto a telephone line to a called party. Windings for a common bridge relay connect in circuit with each telephone line and diodes shunt the windings so the relay responds only to signals of one polarity. The contacts of the common bridge relay, therefore, reflect the duty cycle of the signal through the common bridge relay and this duty cycle varies for each telephone line when the called party goes off-hook during a ringing burst. The common bridge relay produces a supervisory signal that is sampled and transferred, for each telephone line, to a corresponding port storage area. A single ring trip analyzer sequentially monitors each port storage means during a ringing operating to determine whether the supervisory signal indicates that the called instrument has been taken off-hook. When this occurs, the analyzer updates control information in a corresponding port storage area to control the state of a ring delay. The ring delay then opens and removes the ringing signal from the corresponding telephone line. The analyzer also detects off-hook status during intervals between successive ringing bursts and during normal operations when a subscriber lifts a telephone off-hook in order to place a call. The operation of this circuit requires a sufficient direct current component in the ringing signal to operate a relay, and is thus not suitable for an NTU where the supply of power is limited.
U.S. Pat. No. 4,731,828 (Basehore) discloses a ring trip detection circuit in a telecommunications exchange wherein the ringing signal applied to a customer line is limited to provide a square wave output. Since when the customers instrument is xe2x80x9con hookxe2x80x9d virtually no direct current flows in the line, the square wave output has substantial 50xe2x80x9450 duty cycle and when the customer instrument goes xe2x80x9coff hookxe2x80x9d the DC alters that duty cycle, by sampling the square wave signal and counting in one direction for positive samples and in the opposite direction for negative samples, any significant change in the duty cycle causes a counter to exceed a ring-trip threshold fairly rapidly. The operation of this circuit also requires a sufficient direct current component in the ringing signal to operate a relay, and is thus not suitable for an NTU where the supply of power is limited. In addition, each telephone line requires its own ring trip detection circuit which is costly.
U.S. Pat. No. 4,827,503 (Takato et al.) discloses a constant current circuit, including a constant current source which draws a current and a constant current source which sends a current, is connected to a two-wire subscriber line. A ringing signal made up of constant stream of positive and negative current signals is transmitted to the telephone line. The off-hook condition of each telephone line is detected by comparing the output voltage across a resistor with a reference voltage for ring trip. The operation of this circuit also requires a sufficient direct current component in the ringing signal to operate a relay, and is thus not suitable for an NTU where the supply of power is limited. In addition, each telephone line requires its own ring trip detection circuit which is costly.
There is therefore a need for a ring trip detection circuit that does not require the supply of a sufficient direct current source for operation during a ringing signal, and does not ring an individual ring trip detector for each subscriber line.
It is therefore an object of the present invention to provide a single ring trip detector circuit and method for a subscriber line interface circuit having a plurality of subscriber telephone lines connected thereto and which allows the AC ringing voltage to be quickly removed from a subscriber line which goes off-hook, without halting the ring signal to other subscriber lines which remain on-hook and to which a call is also directed.
According to a first aspect of the invention there is provided a method for ring trip detection for use with a plurality of subscriber line interface circuits (SLIC) each having at least one Customer Premises Equipment (CPE) connected thereto and being coupled to a ring generator for feeding a ringing voltage to said plurality of subscriber line interface circuits, the method comprising:
(a) comparing a function of the ringing voltage with a predetermined reference so as to detect when one or more CPEs of the one or more ringing subscriber lines goes off-hook,
(b) disconnecting each of the plurality of subscriber lines from the ring generator,
(c) detecting for each of the disconnected subscriber lines whether it has a CPE in an off-hook condition, and if not
(d) reconnecting the disconnected subscriber line to the ring generator.
According to a second aspect of the invention there is provided a ring trip detector circuit for a plurality of subscriber line interface circuits (SLIC) each having at least one customer premises equipment (CPE) connected thereto and being coupled to a ring generator for feeding a ringing voltage to said plurality of subscriber line interface circuit, the ring trip detector circuit comprising:
a comparator for comparing a function of the ringing voltage with a predetermined reference so as to detect when at least one CPE of the at least one ringing subscriber goes off-hook and producing a comparator detect signal,
a control circuit coupled to the comparator and being responsive to the comparator detect signal for producing a disconnect signal,
a switching circuit coupled to the control circuit and responsive to the disconnect signal for disconnecting each of the plurality of subscriber lines from the ring generator, and
a respective off hook detector for each subscriber line being coupled to the control circuit for detecting an off hook condition for the respective subscriber line in an absence of said ringing voltage;
the control circuit being responsive to at least one of the off hook detectors for reconnecting a respective ringing subscriber telephone line that is not off hook.
Preferably, the control circuit is realized in software using a suitably programmed microprocessor having a sufficiently fast clock rate to allow rapid disconnection and sampling of the subscriber units, so that the ringing signal can be disconnected from a called subscriber unit which goes off-hook in less than 200 ms.